Coral Spawning

Robert H. Richmond, UOG Marine Lab

Summer means different things to different groups. For students and teachers, its vacation time, for construction workers, hot and sweaty conditions, for the corals of Micronesia, its time to spawn and produce the larvae that will seed reefs for the years to come... Corals are animals, and like all other life forms, must reproduce to maintain or increase their numbers. Some coral colonies die each year from both natural and man-made causes, including typhoons, crown-of-thorns starfish, coastal runoff, sedimentation, anchor damage, and sewage impact. Unless these corals are replaced through successful reproduction followed by settlement and metamorphosis of the coral seed called planula larvae, the reef goes into decline, and the important functions and benefits of the coral reef are lost.

The majority of Micronesia's nearly 300 species of coral are simultaneous hermaphrodites, which means they are both male and female, containing both eggs and sperm, at the same time. In the recesses of the coral polyps, eggs begin to develop as early as January each year, and along with the sperm, are fully ripe by June or July. Over the past several years, our research group at the University of Guam Marine Lab has found that many of the corals of Micronesia spawn (release their eggs and sperm) 7 - 10 days after the July full moon.

On the night of spawning, soon after sundown, the flower-like polyps which make up the living coral colony begin to expand, as sperm packets are moved up from a space within the underlying coral skeleton. Soon, bright orange or red eggs are brought up and "glued" to the individual sperm bundles, until the sperm is completely surrounded. This is a nice trick, as these clusters of 9 - 190 eggs around a central sperm core, float to the surface due to the high fat content of the eggs. In this way, eggs and sperm from different colonies have a chance to intermingle at the ocean's surface, and if the combination is right, fertilization will take place.

Our research has shown coral eggs can distinguish among the different kinds of sperm present during the mass-spawning. They usually reject sperm which originate from their own parent colony but readily accept sperm from a different individual of the same species. We have also been able to get sperm from one species to fertilize eggs of related but different species. This is called hybridization, and such crosses may be one reason for the wide variety of coral forms on the reef.

On the pollution front, we've found that freshwater runoff alone can have disastrous effects on coral fertilization rates, with a 15% drop in seawater salinity causing nearly a 90% drop in coral fertilization rate. Add red soil to the runoff, and the situation gets worse. We are also beginning to test the effects of pesticides on coral reproduction, as we need to know if these chemicals will be a problem, based on our present knowledge that chemical cues are very important to the success of corals during their once-a-year breeding period.

One of the most exciting aspects of this research is our experiments with the mass-culture of coral larvae for the aquarium trade, monitoring and tests of reseeding damaged reefs. We are able to raise larvae by the thousands, and use the cultivated corals for testing the effects of pollutants and in reseeding trials.

Every year, we are learning more about corals, how they live, and how they replenish their numbers. This knowledge will help in preserving these precious resources, as well as allow us to improve the opportunities for recovery when damage occurs. However, we can already say, beyond any doubt, that prevention is the key. No matter how successful reseeding is, we cannot speed up the growth rate of corals. Large coral colonies cannot be replaced in less than the several hundred years it took them to attain that size. Coral reefs are economically, culturally and aesthetically important to the Islands of Micronesia. Through a better understanding of how they work, we can do a better job of protecting them from unnecessary damage.


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